(1) Field of the Invention
The present invention relates to computer channel access circuits for input-output devices and more particularly to a circuit which controls computer channel access time of input-output devices.
(2) Description of the Prior Art
Input-output devices access a computer through an input-output device controller or channel. This controller transmits access requests and data from the device to the computer and it transmits data to the device from the computer. Multiple devices, such as local and remote teletypewriters, can be connected to one controller to receive data since they will both print the same data. However, only one teletypewriter can transmit data to a controller at a time since the probability that they will both be transmitting the same data at the same time is infinitesimally small.
If two teletypewriters with input capability were connected to one controller, garbled data would be transmitted to the computer if both teletypewriters were transmitting at the same time. A lockout circuit could be provided such that the teletypewriters accessing the controller first, would lockout the other teletypewriter until its communication with the computer was complete. However, this could result in the other teletypewriter being locked out indefinitely.
This problem has been solved in the prior art by connecting an additional controller to the computer for each additional teletypewriter with input capability. However, such a technique is expensive and unnecessary since it requires an additional controller for a remote teletypewriter which will only occasionally require access to the computer.
Accordingly, it is an object of the present invention to provide a novel, low cost technique of providing computer access capability to multiple input-output devices connected to a single device controller.